1. Field of the Invention
The present invention relates to apparatus for extruding uniform coverings of plastic insulating materials upon filamentary cores, and more particularly, to apparatus for extruding first and second concentric sheaths of plastic insulating material coaxially around a longitudinally advancing filamentary core.
2. Technical Considerations and Prior Art
In the manufacture of plastic insulated wires, it is frequently desirable to provide the wire with two or more concentric uniform tubular sheaths coaxial with the wire. For example, in dual insulated wire, an inner sheath, which may be a soft plastic, or foam plastic, provides the primary insulating covering for the wire while an outer sheath may be a harder or more dense plastic to provide a protective covering for the inner sheath and the core.
Several problems have arisen when it is desired to extrude two concentric insulating sheaths of plastic material around a filamentary core, especially when the inner sheath is an expanded or foam-type plastic, for example, expanded polyethylene or polypropylene.
One method of extruding a dual insulating covering on a filamentary core utilizes a tandem arrangement of extruders one extruder for each sheath to be applied to the core in an extrusion die therein. While this is the simplest method of applying dual insulation, it is difficult to control the eccentricity of successively applied coverings thus giving rise to nonuniform capacitance variations coaxially in the cross sectional plane of the sheathed conductor.
Additionally, the tandem extrusion method, when utilized to extrude an expandable plastic such as polyethylene foam, gives rise to an inherent problem therein, that being the plating out in the forming portion of the die, of the heat-decomposible blowing agent admixed with the plastic compound, which requires the extrusion pressure to be increased to achieve a constant flow rate. Furthermore the residue formed in the die eventually leads to eccentricity of the sheaths and out-of-roundness of the product. Also the second sheath must be applied by tubular type extrusion tooling, e.g. in the second extruder, the plastic material cannot contact the primary insulated core until it has exited or nearly exited the die, since this is the only way to insure that sufficient clearance is provided in the second extruder to pass the insulated core. Since tubular extrusion is limited to lower line speeds, the tandem method results in lower production than can be achieved with a pressure method, where the plastic compound contacts the core in the die.
An improved prior art method of extruding dual concentric plastic sheaths onto an axially advancing conductor utilizes a single extrusion chamber in which first and second spaced, coaxially aligned dies pass the core sequentially. Plastic material for the inner sheath is delivered from a first extrusion bore to the first die where the plastic material is applied to and formed around the core. The insulated core is thereafter passed through the second die, to which the second plastic material is delivered from a second extrusion bore to impinge upon and be formed around the insulated core.
The two-die method permits higher line speeds than the tandem method, but requires a critical coaxial alignment of the dies to prevent eccentricities between the insulating sheaths. Furthermore, when an expanded plastic is utilized for one or both sheaths, plating out of the blowing agent on one or both dies is still a problem. But the most serious defect of the two-die method is that the pressures in the dies are cumulative, making it difficult to control the blowing of the expanded plastic.
In still another prior art method, the plastic material for the outer sheath of a dual insulated wire is bled around the entrance portion of the forming die in the extrusion passageway before the insulating material for the first sheath is compressed around the advancing core such that the second plastic forms a barrier between the forming die and the first sheath.
This method prevents plating out of the blowing agent where the inner sheath is an expanded plastic and the outer sheath is a solid plastic, but, since the plastic material for both sheaths communicate with one another directly before the flow of at least the plastic for the inner sheath is permitted to develop, irregularities and nonuniform skin thickness can result. Furthermore, if there are substantial differences in the melting point temperatures of the materials, the hotter plastic may cause the cooler plastic to burn, or the cooler plastic may reduce the temperature of the hotter plastic below its melting point causing premature solidification thereof, producing lumps therein.
In still another prior art method the outer sheath is formed as the core having the inner sheath already formed thereon issues from the die in a single extrusion chamber. The outer sheath is formed by the plastic material therefor being forced onto the inner sheath in two or more streams. This can result in "knit" lines being formed on the outer sheath where the streams converge. The localized pressure from the streams may also deform the inner sheath and the core.
There exists therefore, a need for a method of and apparatus for extruding dual concentric plastic sheaths onto a moving elongated core at high line speeds and low pressures wherein plating out of the blowing agent for expanded plastics is eliminated, or minimized and better control of expansion is obtained, wherein the temperature differences of the plastic materials present no problems, and wherein the inner and outer sheaths are concentric and are of uniform thickness.